TheSamba.com :: View topic - What is the limiting factor, venturi or throttle bore size?

Mon Feb 23, 2009 10:31 pm

Hey everybody, I have a quick question about carburetor throttle bore size VS venturi size.

I have a 1915 engine in my bug and I have several sets of Dellorto carburetors. The first set are 45s with 38 vents. While driving with them the bottom end is a bit soggy but the top end feels great. It revs out to 5500 rpm very nicely. The other set are 36 dells with 34 vents. The bottom end is great but after 4500 rpms the power just drops off.

My question is this, if I put the 34 vents into the 45 dells will the top end power be limited at 4500 rpm still? Is the Venturi the limiting factor or only part of the equation?

For what it is worth I have a LM1 and have the jetting pretty close. Both carbs are showing 12.3 to 12.6 afr during full throttle tests.

Thanks in advance,

Jeremy

Mon Feb 23, 2009 11:57 pm

Oddly enough, I have 40 Dells w/ 34 vents on my 2007, and can hit 5500rpm easily. I would say fisrt the vent size will be the limit, then the body size.

Tue Feb 24, 2009 1:02 am

There are quite a few posts on this but the main limiting factor is the size of the carb body

This limits the amount of air to the venturi

a 32mm vent on a DRLA 36 gets more air in a DRLA 40 and is more efficient and will allow greater a rev range. Similarly a 36mm vent in a 40 IDF isn't as efficient as a in a 44 IDF

The more efficient a venturi is, the greater port velocity the air can achieve before odd things happen and the revs fall off

Greater port velocity equals higher revs in a roundabout fashion

Tue Feb 24, 2009 8:50 am

full throttle "tests" your main cct of the carb. This has no relation to your idle cct.

Also bare in mind that TOO MUCH port velocity creates a sonic choke.

Since you haven't mentioned cam size, gear ratio, weight of vehicle, tire diameter those also have alot to do with bottom end grunt.

Throttle plate and venturi size are the limiting factor. The hard part it to match the carbs with the engine displacement.

I would think a 40mm throttle plate would match a 1915cc in terms of meeting a compromise of good bottom end and acceptable top end. Of coarse this also depends ported heads, properly ported intake manifolds etc etc.

NO cut and dry answer really. If your operating in the higher rpms for max HP you may need to settle for a slower velocity at low rpms (45 dells)

Tue Feb 24, 2009 9:02 am

Your 34 vents will fit 36 and 40mm DRLAs, not 45 DRLAs.

What cam and heads you have will affect things too.

Tue Feb 24, 2009 9:43 am

Alan_U wrote:

Also bare in mind that TOO MUCH port velocity creates a sonic choke.

I read quite a lot about this recently (didn't understand a hell of a lot of it though!)

About 0.6 mach, or about 450 miles per hour is when sonic choke begins to reduce volumetric efficiency! :shock:

Although this is a huge number, it is not outside the realms of possibility. It's possible to do a ball park calc on this but it's dependant on your cylinder size and piston velocity ratio'd to your ports etc.

Alan_U wrote:
Throttle plate and venturi size are the limiting factor. The hard part it to match the carbs with the engine displacement.

I would think a 40mm throttle plate would match a 1915cc in terms of meeting a compromise of good bottom end and acceptable top end. Of coarse this also depends ported heads, properly ported intake manifolds etc etc.

NO cut and dry answer really. If your operating in the higher rpms for max HP you may need to settle for a slower velocity at low rpms (45 dells)

Very true

Almost any carb will run on an engine but say for example you tried a 34 PICT 3 on a 2276. It'd probably not do much more than Idle and get very hot very quickly as the carb wouldn't be able to supply enough air to all four cylinders without it running enormously lean, if at all.

Choking the flow will reduce your power and rev range capability so, depending on what you want the engine to do, you need to pick an appropriate choke diameter.

The 34 vents won't work quite as effectively as you'd like in the 36's as there will be very little pressure differential in the venturi's throat compared with a larger carb body. The pressure drop is created by increasing the velocity of the air and with only a slight restriction, the pressure drop will be barely enough to suck gas at low revs and at higher revs, the effect will be hardly noticeable, hence the drop off. You'd get a better response with a 32 venturi in the 36's as the pressure drop will work up to nearer 5000 revs. Mine crap out at 4800 with 32's but on a 2017 with small ports. bigger ports and less displacement with your 1915 should allow a higher range to be achieved.

Personally, As you've got them, I'd stick the 45's on and just go and get some 34 chokes that will fit them which would work quite nicely up the top end as well as the bottom.

Tue Feb 24, 2009 10:03 am

Marv,

IIRC when a head's port velocity goes beyond 350 fps it starts to choke.

In the carb I'd expect its more complicated since the velocity stack, airhorn, venturi, booster(aux) venturi, intake manifold etc are more variables before it even gets to the head.

Since the venturi is the main "choke" thats the big bottle neck.

Unfortunately no cut and dry answers since every individual has there interpretation of what driveability is.

Tue Feb 24, 2009 10:09 am

Alan_U wrote: Marv,

IIRC when a head's port velocity goes beyond 350 fps it starts to choke.

Cheers Alan, You learn something new every day :D

Tue Feb 24, 2009 6:58 pm

Thanks everybody for the replies.

Alan_U wrote:

Since you haven't mentioned cam size, gear ratio, weight of vehicle, tire diameter those also have alot to do with bottom end grunt.

I left this out on purpose as I don't want the discussion to move towards engine building but will be happy to include it if that would make a difference.

Alan_U wrote:
Throttle plate and venturi size are the limiting factor. The hard part it to match the carbs with the engine displacement.

So is it throttle plate first then venturi size?

Alan_U wrote:
I would think a 40mm throttle plate would match a 1915cc in terms of meeting a compromise of good bottom end and acceptable top end. Of coarse this also depends ported heads, properly ported intake manifolds etc etc.

I only have one 40mm Dell. I thought it would be cheaper to get the 45s optimized than to buy another carburetor. I will give it a try and find out.

Tue Feb 24, 2009 7:02 pm

mharney wrote: Your 34 vents will fit 36 and 40mm DRLAs, not 45 DRLAs.

Mark,

Are you thinking about 48 dells here? I have a 36, 40 and a 45 dellorto in front of me as well as 30, 34 and 38 vents, all vents fit into each carb body just fine. The only problem I could see is in the taper at the bottom of the vent may be matched for each carb body size.

Tue Feb 24, 2009 7:10 pm

Marv [UK] wrote:

Choking the flow will reduce your power and rev range capability so, depending on what you want the engine to do, you need to pick an appropriate choke diameter.

The 34 vents won't work quite as effectively as you'd like in the 36's as there will be very little pressure differential in the venturi's throat compared with a larger carb body. The pressure drop is created by increasing the velocity of the air and with only a slight restriction, the pressure drop will be barely enough to suck gas at low revs and at higher revs, the effect will be hardly noticeable, hence the drop off. You'd get a better response with a 32 venturi in the 36's as the pressure drop will work up to nearer 5000 revs. Mine crap out at 4800 with 32's but on a 2017 with small ports. bigger ports and less displacement with your 1915 should allow a higher range to be achieved.

I don't have any 32s around but do have some 30s. It would be fun to test this, I may give this a shot. Depending on the result of that test I may try to scare up as set of 32s.

Marv [UK] wrote:
Personally, As you've got them, I'd stick the 45's on and just go and get some 34 chokes that will fit them which would work quite nicely up the top end as well as the bottom.

Sounds like a plan as I already have these parts. I know I will need to move my main jets down with the move down in vents.

Tue Feb 24, 2009 7:13 pm

Marv [UK] wrote: Almost any carb will run on an engine but say for example you tried a 34 PICT 3 on a 2276. It'd probably not do much more than Idle and get very hot very quickly as the carb wouldn't be able to supply enough air to all four cylinders without it running enormously lean, if at all.

Why would using a 34PICT on a 2276 cc engine cause it to run lean? The fuel mixture is controlled by the jetting. The vacuum pulse would be large and give you great low RPM response. Using a large displacement engine with a small carb could result in a lower density fuel mixture at higher RPM as the volumetric efficiency would drop off. I would expect that such a combination would be great at low RPM and then just hit a wall at higher RPM where the power would drop off.

Scott Novak

Tue Feb 24, 2009 7:14 pm

Since we are already talking about vents I have a question about secondary vents. See this picture bellow, what is the difference between them, is one style preferred over the other? Of the ones on the left, some of mine are marked with a 1 and some are marked with a 2.
Thanks in advance for any responses.

Tue Feb 24, 2009 8:00 pm

Scott Novak wrote: Marv [UK] wrote: Almost any carb will run on an engine but say for example you tried a 34 PICT 3 on a 2276. It'd probably not do much more than Idle and get very hot very quickly as the carb wouldn't be able to supply enough air to all four cylinders without it running enormously lean, if at all.

Why would using a 34PICT on a 2276 cc engine cause it to run lean? The fuel mixture is controlled by the jetting. The vacuum pulse would be large and give you great low RPM response. Using a large displacement engine with a small carb could result in a lower density fuel mixture at higher RPM as the volumetric efficiency would drop off. I would expect that such a combination would be great at low RPM and then just hit a wall at higher RPM where the power would drop off.

Scott Novak

The reserve in the jet well, Scott. As the demand on the fuel goes up, the dynamic height of the fuel in the jet well will go down. The rate at which the jet can supply fuel to the well will be "outrun" by the demand for fuel, with the air correction jet mobilizing the fuel up the well faster than the jet can keep up. A larger main jet would have to be used to keep the opening large enough to supply enough fuel. Also consider that as you go into tubulent flow, the boundary layer grows and the nozzle will "see" a different depression. The mass of airflow will head for open range.

Tue Feb 24, 2009 10:15 pm

mharney wrote: The reserve in the jet well, Scott. As the demand on the fuel goes up, the dynamic height of the fuel in the jet well will go down. The rate at which the jet can supply fuel to the well will be "outrun" by the demand for fuel, with the air correction jet mobilizing the fuel up the well faster than the jet can keep up. A larger main jet would have to be used to keep the opening large enough to supply enough fuel. Also consider that as you go into tubulent flow, the boundary layer grows and the nozzle will "see" a different depression. The mass of airflow will head for open range.

So a 34 PICT might not be capable of working correctly an a very large displacement engine. But if a 34 mm carb was designed for higher air flow it should be able to work properly, but with RPM limitations?

Gene Berg claimed many of his customers had used up to 2,200 cc engines with the stock carb and they ran well and reliably. I'm not sure what kind of modifications he may have made to those carbs.

Scott Novak

Tue Feb 24, 2009 11:12 pm

It's all about getting the carb to provide what you want under all flow conditions. At some point something fails to increase, and in the case of the 34PICT carb on a big engine, the main jet is likely to be done first, regardless of other affects, so going to a bigger jet is the solution. The float bowl and jet well are nothing more than a rate limited manometer with something sucking fluid out above the meniscus on one side, with the addition of an emulsion tube to help mobilize the fluid along the column wall. The size, depth, and gauge of the tube determines chiefly the jet well reserve, static by displacement, and dynamic by what it does with the air coming down. The holes in form, size and number affect the dynamic reserve under different conditions, as well as rate of discharge. At some point you suck a lot of air down the tube, and demand fuel volume at a rate that is beyond what the system can give up. One thing is the controlling variable, and I'd bet it to be the main jet. The larger main jet will increase the dynamic reserve volume for sure. The only reason I go through this is to illustrate that there is a limiting factor in a rather complex system.

Re: Berg - Realize that also fuels were different back then.. no ethanol to require increased volume against higher viscosity. Success using a stock Solex on a 2200 cc engine may be limited to the torque range, to say the least anyway. That's a small carb, and is about like trying to run around the block breathing through a straw. I can run 28 and 30 PICTs on my 1600 and even with that it's a docile combo in comparison. Runs great though..

Larger engines will require a bit more air at idle too, and with a 30PICT3 or 34PICT3 in place, it's save to say that the larger engine will require more turns on the volume screw. That may just be a saving grace of the these on larger engines too, that it's idle air flow is not depending on throttle plate position, and doesn't suffer exposed progression, like the 28 and 30PICT1 and 30PICT2. Not that you can't remedy that as well on the latter models. :idea:

Mon Mar 02, 2009 6:47 pm

Just a quick update. I was able to install 34 mm vents into my 45 Dells and take the car for a test drive. It easily revs out to 5500 rps now and the low end no longer feels soggy. So far the AFR seems to be more stable with this set up. I went down on the main jet size when I went to the smaller vents, I may try a smaller idle jet as well and see how that goes.

So what I learned is that the throttle bore plays a major part in the flow of the carb and that the vent is not the limiting factor.

Mon Mar 02, 2009 7:36 pm

What you should also know is that you observed two extremes. 36 DRLA was never meant to have 34mm vents in it.

The 40 DRLA is a great carb with 34 vents, and would work well also.

The best match is usually one where you have about 80% vent size to bore size ratio.

Mon Mar 02, 2009 10:50 pm

I have never heard of the 80% rule before, good stuff. I was at almost 95% with the 36 dells.

Wed Mar 04, 2009 10:38 pm

j-dub wrote: Hey everybody, I have a quick question about carburetor throttle bore size VS venturi size.

I have a 1915 engine in my bug and I have several sets of Dellorto carburetors. The first set are 45s with 38 vents. While driving with them the bottom end is a bit soggy but the top end feels great. It revs out to 5500 rpm very nicely. The other set are 36 dells with 34 vents. The bottom end is great but after 4500 rpms the power just drops off.

My question is this, if I put the 34 vents into the 45 dells will the top end power be limited at 4500 rpm still? Is the Venturi the limiting factor or only part of the equation?

For what it is worth I have a LM1 and have the jetting pretty close. Both carbs are showing 12.3 to 12.6 afr during full throttle tests.

Thanks in advance,

Jeremy

You are looking for 16 pounds of air under a light load. When under full power a mixture is about 12.5 pounds of air per pound of fuel. Thats for a complete burning in all cylinders